Synthesis of polymer emulsions in an aqueous system is conventionally performed by polymerization of monomers with emulsifiers in either a batch polymerization or a seeded polymerization process.
In the batch polymerization process all of the ingredients are charged to a reactor vessel followed by an initiator to make a final latex product.
In the seeded polymerization process, either an external or an in-situ seed is used. The external seed is commonly a very small particle size latex made by the batch polymerization process. This latex is stored and small quantities are then used as needed as a seed to polymerize and grow larger particle size latex products.
The in-situ seed preparation process is the first stage (Stage 1) of a continuous polymerization process where water, emulsifiers, chelates and a small portion of the recipe's monomer, alone or with a comonomer, are polymerized to form the desired number of seed polymer particles. All other conditions being equal, the amount of monomer used for seed determines the number and size of seed particles formed and the particle growth of the final latex product. The seed, once formed, is followed by a second stage (Stage 2) of successive additions of the remaining recipe monomers to form the final latex product. Seed monomers plus additional monomers total 100 parts by weight.
In the emulsion polymerization of in-situ prepared seed polymer, the seed cannot be comparatively freely checked and controlled because the preparation of the seed is an integral part of the process for making the final product. Accordingly, as part of an on-going process, correction of the properties of the seed polymer cannot be made. Therefore, process consistency is critical. External seed polymerization has an advantage over in-situ seed polymerization in that the particle size of the seed polymers can be comparatively freely checked and controlled thus assuring controlled particle growth in the final latex product. However, extra production storage tanks are required and seed storage stability can become critical.
In conventional seeded polymerization processes the reactions are typically carried out in the presence of seed polymer particles having a molecular weight of, e.g., 100,000 or more of weight average molecular weight, while adding polymerizable monomers to the reaction system. During the seeded polymerization process, the essential ingredients are monomer, emulsifying agent, water and a water soluble initiator. Typically, the monomer is dispersed in the aqueous phase by the emulsifying agent. The reaction is then initiated by the water soluble initiator.
Three distinct phases can be identified during Stage 1 of the seeded polymerization process. In Phase 1 of Stage 1 of the formation of, for example, polystyrene seed, styrene is present in large monomer droplets and in monomer swollen emulsifier micelles. A small amount (0.054% at, 153.degree. F. (Fahrenheit)) is dissolved in the water phase. Radicals generated by the initiator diffuse through the water phase and react with the styrene in the water phase and in the micelles to initiate polymerization. Because the total surface of the micelles is very large compared to that of the monomer droplets, initiation occurs almost exclusively in the micelles. Any growing polymer molecules in the water phase rapidly become insoluble and adsorb emulsifier in order to maintain colloidal stability. The large monomer droplets serve as a reservoir of styrene that continually diffuses into the water as styrene from the water is absorbed into the micelles forming the growing polymer particles. When the number and size of growing particles swollen with styrene reaches the point at which all the emulsifier has been adsorbed on these particles, all new particle formation ceases.
In Phase 2 of Stage 1 of the seeded polymerization process, the seed polymerization rate is nearly constant as styrene diffuses from the droplets through the water to the growing polymer particles. This phase continues until all of the monomer droplets are depleted.
During Phase 3 of Stage 1 of the seeded polymerization process, the reaction occurs only in the monomer swollen polymer particles. Near the end of the polymerization, the reaction rate decreases as the styrene concentration decreases.
In the in-situ seed process, during Stage 2, the remaining recipe monomers to form the final latex must be introduced into the reactor while the seed polymer is still active. Stage 2 may be started when the seed polymer has reached 80-90 percent conversion.
It has been an accepted practice to co-polymerize monomeric acids such as acrylic, methacrylic, itaconic and the like with aromatic and aliphatic monomers to introduce carboxyl functionality and strengthen the finished polymer. However, it will be appreciated that this has the disadvantage of either adversely affecting colloidal stability or the conversion level of these monomers either in Stage 1 (i.e., during seed formation) or Stage 2 (i.e., during monomer feed).
It will also be appreciated from the foregoing that there is a significant need for an improved process of preparing seed polymer that overcomes the problems of the prior art. Accordingly, it is an object of the present invention to provide an improved process of preparing seed polymer. Another object of the present invention is to provide a process of preparing seed polymer that eliminates or reduces the induction period thereby reducing initiation and particle size variability. Yet another object of the present invention is to provide a process of preparing a seed polymer having a shortened seed cycle time. Another object of the present invention is to provide a process of preparing a seed polymer with improved colloidal stability and with improved conversion of monomer to seed polymer to improve the final latex product. Still another object of the present invention is to provide a process of preparing a seed polymer wherein the level of more expensive polymerizable monomeric acids is reduced. Another object of the present invention is to provide a method of lowering the residue of the seed polymer and of the final latex product. Another object of the present invention is to provide a method of improving the calcium ion tolerance of the seed polymer and of the polymer latex. Yet another object of the present invention is to provide a process of preparing seed polymer that is simple and economical to practice.